A flyback converter circuit includes a transformer configured to provide voltage conversion, with galvanic isolation, between an input voltage and an output voltage. Flyback converters have been used both for AC/DC and DC/DC voltage conversion. Galvanic isolation is commonly used for circuit safety purposes, to isolate two or more electric circuits that must communicate, but that have grounds that may be at different potentials. It also can be an effective technique to prevent unwanted current from flowing between electrical components sharing aground conductor.
FIGS. 1A-1B are illustrative circuit diagrams representing a known flyback converter 102 in an on-state (FIG. 1A) and an off-state (FIG. 1B). A primary side 104 of the flyback converter 102 includes an input voltage source Vin, a transformer primary winding 108 and a switch 110 to controllably couple and decouple the input voltage Vin to and from the primary winding 108. A secondary side 112 of the converter 102 includes a transformer secondary winding 114, a capacitor 116 coupled in parallel with the secondary side winding 114, and a diode coupled 118 to permit unidirectional current flow between the secondary winding 114 and the capacitor 116. An output load 120 is coupled in parallel with the capacitor.
Referring to FIG. 1A, when the switch 110 is closed, the primary winding 108 is directly coupled to the input voltage source Vin. A current within the primary winding 108 and magnetic flux in the transformer, which includes the primary and secondary windings, increases gradually, storing energy in the transformer. A voltage induced in the secondary winding 114 is negative, so the diode 118 is reverse-biased (i.e., blocked) and no current flows in the secondary winding 114. During a time interval while the switch 110 is closed, the capacitor 116 supplies energy to an output load 120.
Referring to FIG. 1B, when the switch 110 is open, the primary winding 108 is decoupled from the input voltage source Vin. Current within the primary winding 108 and magnetic flux drop. A voltage induced in the secondary winding 114 is positive, resulting in forward-bias of the diode 118, which allows current to flow from the secondary winding 114 to charge the capacitor 116. Energy transferred from the transformer thereby recharges the capacitor 116, which supplies an output voltage to the load 120.
Often, an electrically isolated communication medium (not shown) is used to transmit control signals between control circuitry disposed on a primary side 104 of the converter 102, and control circuitry disposed on a secondary side 112 of the converter 102. The control signals ordinarily are used to control timing of the opening and closing of the switch 110 so as to regulate voltage conversion to maintain a desired output voltage level, for example. The electrically isolated communication medium may include opto-couplers, an inductive couplers, or capacitive couplers, for example.